Roof flashing is conventionally used to provide a waterproof and weather resistant seal around pipes, vent stacks and other elongate members projecting from roofs. The flashing is generally constructed from sheet metal, flexible moulded rubber or other synthetic material formed in an appropriate shape to encircle such projecting members, and is normally made to extend slightly above the level of the roof, to limit the infiltration of precipitation. In flat roofs, where it is possible that standing water might accumulate, this is particularly important. In some applications, including exhaust vent stacks, cap members are mounted on top of the roof flashing and vent stack. In other applications, including support members for roof mounted apparatus, it is critical to provide a long lasting, water tight seal of any gap that exists between the flashing and the projecting member. Commonly, such gaps are sealed by caulking with resilient materials, such as silicon.
Caulking is generally effective in this use. However, the use of caulking requires that close tolerances be maintained to ensure that the gap to be sealed is sufficiently narrow for a caulk seal to be established. Thus, where vent pipes or stacks of differing outside diameters are to be utilized, different flashings matching the differing vent pipes or stacks must be purchased and installed. The requirement for matching flashings also makes it difficult to accommodate modifications to designs made during construction, which often occur. Equally problematic is the gradual and often eventual breakdown of the integrity of the caulking seal caused by movement of the roof flashing and stack members due to expansion and contraction of their components because of seasonal temperature fluctuations. Further, caulking can be difficult to work with, can have a limited effective lifetime, and can be expensive. Moreover, its installation is labour intensive.
It is known in the prior art for grommet seals to be utilized to seal gaps between pipes or stacks and flashing, instead of caulking. An example of a grommet seal for use with a roof flashing is found in U.S. Pat. No. 5,802,787, issued Sep. 8, 1998, to Kenneth Thaler, which patent is directed to a resilient grommet for forming a flexible seal between a roof flashing and a co-axially disposed elongate member, and is incorporated herein by reference. The resilient grommet therein described forms a mushroom shaped head on the top of the flashing to effectively shed precipitation and avoid accumulation or pooling of water and ice on the flashing. The bottom portion of the grommet provides a rounded edge to collect-form droplets, which are quickly shed from the assembly. The grommet is formed of homogenous elastomeric material which resiliently surrounds an outwardly extending flange on the flashing. The resilient grommet is stretched over the flange and snaps back into its original shape when installed. The grommet is easily properly positioned on the flange during installation and is not easily dislodged, due to its resilience and its close fitting engagement with the flange.
It is evident that the use of resilient grommets has advantages over the use of caulking in the sealing of gaps between flashing and elongate members, such as vent pipes or stacks. Firstly, a variety of grommets can be produced, each adapted to fit a common flashing, but having varying internal diameters, to seal against vent pipes or stacks of differing external diameters. This allows for common flashings to be utilized during construction, cutting down on cost and waste, and also allows for convenient changes to the vent pipes or stacks during construction. Similarly, where final designs have not yet been made, the use of grommets as described allows a flashing to be mounted, and the roof sealed therearound, while final design decisions are made, with confidence that it will not be necessary to replace the flashing and reseal the roof if a vent pipe or stack of a different diameter than that which was expected is ultimately determined to be utilized. Such a feature has benefits, in that certain construction trades, such as drywallers and painters, normally can not proceed until the roof of a structure has been sealed, and might otherwise be required to postpone their activity, slowing down the overall pace of construction, and adding to costs.
However, known prior art roof flashing constructions suffer from a susceptibility to the build up of condensation within the flashing. This may even be exacerbated in roof flashing assemblies having resilient grommet seals. In known prior art roof flashing constructions, an air space is created between the outer surface of the elongate member and the interior surface of the flashing member, which air space is in liquid communication with the inside atmosphere of the structure. This allows moisture in warm air from within the structure to collect, condense and freeze within the flashing during winter months, which collected moisture melts during the spring, eventually leaking into the structure. Because the use of a grommet seal allows for the gap between the elongate member and the inner surface of the flashing to be wider than that which can be accommodated by silicon caulking, a greater volume of frozen moisture may accumulate in such a construction, and in many instances where grommet seals are utilized, the amount of moisture entering the structure in the spring is sufficient to cause the occupants of the building to incorrectly assume that the grommet seal is leaking, resulting in unnecessary roof repairs and an industry perception that grommet seals are unreliable.
In an attempt to alleviate water infiltration into the interior of a building due to condensation, insulation is typically applied to the inside surface of the flashing. Despite the availability of insulated roof flashings, many installers elect, because of cost, to use non-insulated flashings. However, even when insulation is used, it has been found that condensation problems will still persist to an unacceptable degree.